function switchImage(rootPass,imgNum,imgFormat) {

	var idNum = Math.floor((imgNum + 1) * Math.random()); // randam number

	var filePass = rootPass + idNum + '.' + imgFormat; // image pass



	document.getElementById('bg_img').style.backgroundImage = 'url(' + filePass + ')';　// switch background image

}



// run

switchImage('http://odamari.net/img/bg_img/',2,'jpg');